Read double values from text file

Question

I used following function to read double values from a text file and then convert it into a floating point array, but it ends up with unexpected termination during program running. Can anyone tell me what goes wrong here.

float *readFile_double_to_float(char *fileName, int fileSize)
{
    double *array = (double *)malloc(fileSize * sizeof(double *));
    float *array_float = (float *)malloc(fileSize * sizeof(float *));

    FILE *fp = fopen(fileName, "r");
    if (fp == NULL)
    {
        printf("Error: Cannot open file for reading ===> %s\n", fileName);
        exit(1);
    };
    for (int i = 0; i < fileSize; i++)
    {
        fscanf(fp, "%lf", array + i);
    };
    fclose(fp);

    for (int i = 0; i < fileSize; i++)
    {
        array_float[i] = (float)array[i];
    };

    printf("CHECK VAL DOUBLE TO FLOAT : float: %.20f | double: %.20f\n", array_float[0], array[0]);

    printf("FILE READ COMPLETED ===> %s\n", fileName);

    free(array);
    return array_float;
}

sizeof(double *) is the size of a pointer to double, not a double value. On e.g. a 32-bit platform sizeof(double) is typically larger than sizeof(double *), which means you won't allocate enough memory. — Some programmer dude
– Some programmer dude, Commented Jun 26, 2020 at 12:35
And why the temporary double array? Why not read directly into the float array? — Some programmer dude
– Some programmer dude, Commented Jun 26, 2020 at 12:35
regarding: double *array = (double *)malloc(fileSize * sizeof(double *)); and float *array_float = (float *)malloc(fileSize * sizeof(float *)); 1) allocate double or float NOT double * or float * 2) the returned type is void* which can be assigned to any pointer, Casting just clutters the code and is error prone. 3) always check (!=NULL) the returned value to assure the operation was successful. If not successful (==NULL) then call perror( "malloc failed" ); so the user is informed of the problem. — user3629249
– user3629249, Commented Jun 26, 2020 at 13:44

chux · Accepted Answer · 2021-08-30 16:45:56Z

2

Wrong size allocated. @Some programmer dude

//                                          vvvvvvvvvvvvvvvv  size of a pointer
double *array = (double *)malloc(fileSize * sizeof(double *));

Avoid mistakes, drop the cast and allocate to the size of the referenced data. Easier to code right, review and maintain.

double *array = malloc(sizeof *array * fileSize);
//                     ^^^^^^^^^^^^^ Sizeof referenced data

Better code would also test allocation success.

if (array == NULL) {
  fprintf(stderr, "Out of memory");
  return NULL;
}

More informative to use "%e", "%g" or "%a".

//printf("CHECK VAL DOUBLE TO FLOAT : float: %.20f | double: %.20f\n", 
printf("CHECK VAL DOUBLE TO FLOAT : float: % .20e | double: % .20e\n", 
    array_float[0], array[0]);

edited Aug 30, 2021 at 16:45

answered Jun 26, 2020 at 13:49

chux

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